High power pulsed electron beam

ABSTRACT

Apparatus for generating a relativistic annular electron beam with a power in the 1010 watt range utilizing a solid conical cathode of blunted shape and a surrounding truncated anode. A high intensity, pulsed magnetic field guides and focuses the electrons emitted from roughened areas of the cathode by a cold field emission process to form the annular beam.

United States Patent Friedman et a1.

[451 Oct. 24, 1972 [54] HIGH POWER PULSED ELECTRON BEAM [72] Inventors: Mushe Friedman, Washington,

DC; Michael G. Ury, Ithaca, NY.

[73] Assignee: The United States of America as represented by the Secretary of the Navy [22] Filed: Aug. 30, 1971 21 Appl. No.: 175,888

Primary Examiner-Eli Lieberman Assistant Examiner-Saxfield Chatmon, Jr. Attorney-R. S. Sciascia et a1.

57] ABSTRACT Apparatus for generating a relativistic annular electron beam with a power in the l0 watt range utilizing 84 315 531,315 3.5, [52] U S Cl 313/ 315/539 a solid conical cathode of blunted shape and a sur- 51] Int. Cl ..H01j 29/46 rounding truncated anode A high intensity, pulsed [58] Field of Search ..315/3.5, 39.3, 5.39, 3.6; magnetic field guides and focuses the l rons 313/84 emitted from roughened areas of the cathode by a cold field emission process to form the annular beam. [56] References Cited 5 Claims, 2 Drs res UNITED STATES PATENTS 3,489,944 1/1970 Denholm et al. ..315/5.39

CURRENT CURRENT PULSE SOURCE NEGATIVE VOLT PULSE DRIFT SPACE PATENTEDBCI 24 I972 CURRENT SOURCE CURRENT PULSE DRIFT SPACE NEGATIVE VOLT PULSE Fig.1

lOO

wc o l 5 B kilogouss IOO Fig. 2

HIGH POWER PULSED ELECTRON BEAM The present invention relates generally to apparatus for and methods of producing high power electron beams and, more particularly, to an arrangement which can produce a pulsed beam of relativistic electrons capable of power greater than ten gigawatts.

Pulsed beams of relativistic electrons with power between and 10 watts have been generated in the past by applying a high voltage pulse to a low resistance planar diode employing a thin, metallic foil or screen as an accelerating anode through which the beam passes. This field emission technique, in many cases, is unsatisfactory since the anode foil is damaged by the passage of the electron beam and must be replaced periodically. Also, the electrons are scattered as a result of their collision with the foil atoms. A further disadvantage is that there is emission of gas from the foil which may cause arcing in the diode region and contamination of the system. These shortcomings have limited the application of relativistic beams in such areas as high power pulsed microwave production, materials testing, metallurgical analysis and medical diagnosis and therapy.

it is accordingly a primary object of the present invention to provide a pulsed relativistic electron beam with a power in the 10 watt range.

Another object of the present invention is to generate a relativisitc annular electron beam on a repetitive basis.

Another object of the present invention is to provide an electron beam injection gun which forms an annular shaped beam of relativistic electrons whose radius may be controlled by an external magnetic field.

Another object of the present invention is to provide a field emission diode which utilizes a magnetic field to guide the electrons and control the diode impedance.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic illustration of one electron beam injection arrangement; and

F IG. 2 is a plot of magnetic field versus energy at the diode and diode impedance.

Referring now to FIG. 1 of the drawings, it will be seen that the electron beam injection apparatus of the present invention, with its magnetic guiding and focusing feature, includes a cathode l of generally conical shape, with its apex portion 2 blunted. This cathode is made of a nonmagnetic, metallic material. A circumferential area of its slant surface 3 is suitably roughened so as to enhance field emission therefrom. A solid, highly conducting cathode prevents any appreciably penetration of this electrode by the extremely strong, pulse magnetic field utilized in the operation of the apparatus until the time maximum strength is attained. This is, however, not necessary if the magnetic field is shaped so that the flux lines are almost parallel to the cathode.

Surrounding the cathode is an anode 4 inthe shape of a truncated cone. A suitable provision is included in the supporting structure of these two electrodes to permit the spacing a between. their confronting surfaces to be varied. The anode is made of thin, metallic material.

This is to give the structure a sufficiently short L/R time so that the pulsed magnetic field may readily penetrate it with little perturbation. In one embodiment, this electrode was fabricated from 0.030-inch stainless steel sheeting. The thickness is not critical if a DC field is utilized.

Circular magnetic field coils 4 are disposed about the diode structure. All the above components, except for coil 4, together with their complementary structures, are adapted to be enclosed within a suitable evacuated vessel or chamber.

In the operation of the apparatus, a pulsed magnetic field with a rise time of the order of milliseconds and an amplitude above 1 kilogauss is applied to the system by a source of current pulses 5. If the diode structure itself is properly positioned with respect to the magnetic field coils and provided the diffusion time of the magnetic field to the cathode is longer than the rise time of the pulse magnetic field, the magnetic lines of force will be roughly parallel to the cathode slant surface in the channel area between the anode and cathode.

At peak magnetic field, a high voltage pulse of the order of 700 kV and of a duration of 50 nano-seconds is applied to the diode from a source 6. Electrons are emitted from the roughened surface of the cathode by means of a cold field emission process. The magnetic field, however, prevents these electrons from reaching the anode and controls their trajectories to guide them through the cathode-anode channel into, for example, a drift tube and focus them into an annular beam. In this connection, a circular conducting screen 7 may be attached to the anode structure and an additional magnetic coil or series of coils, such as 8, may be included in the system.

It has been found that the radius r of the annular electron beam that is propagated in the drift tube region is related to the radius of the cathode r and to the magnetic field by the following relation:

"e "c( c/ where b is the magnetic field near the cathode and B is the magnetic field in the drift tube.

The following table represents the results obtained FIG. 4 illustrates the result achieved with the 10 cm. diameter cathode with the diode and the drift tube pumped to a common pressure of 5Xl0 Torr. It will be seen that the transmission of energy along the drift tube improves with high magnetic field and approaches percent when B 13 k6. It will also be seen that the diode impedance Z apparently varies as B.

It will be appreciated that the intensely focused electron beam generated may be directed against an appropriate target so as to produce X-rays which may be used for medical diagnosis and therapy or materials testing and analysis. It may also be suitably modulated What is claimed is: 1. Apparatus for producing a high power annular pulsed electron beam comprising, in combination,

a cathode having a generally conical shape with its apex portion blunted, said cathode being of solid construction and made of a non-magnetic, metallic material; an anode having a frusto-conical shape symmetrically positioned about said cathode such that the longitudinal axis of symmetry of said cathode and said anode are co-extensive; a magnetic coil surrounding said anode; means for coupling a high amplitude current pulse to said magnetic coil, the solid and conducting nature of said cathode preventing any appreciable penetration of this cathode by the pulsed magnetic field thereby produced and causing this magnetic field to be parallel to the slant surface of said cathode in the space between said cathode and said anode; and means for driving said cathode highly negative with respect to said anode when said magnetic field has reached its maximum intensity whereby electrons are emitted from said cathode in a cold emission process and thereafter deflected and focused by said magnetic field into a high-power, annular, pulsed electron beam.

2. In an arrangement as defined in claim 1 wherein said cathode has a circumferential portion of its outer surface roughened so as to enhance electron emission therefrom when said cathode is driven highly negative with respect to said anode.

3. In an arrangement as defined in claim 1,

wherein said anode is made of thin metallic material so as to permit the magnetic field to penetrate it without distortion.

4. In an arrangement as defined in claim 1, a cylindrical screen,

one end of said screen being connected to the apex portion of said anode and maintained at the same electrical potential as said anode.

5. In an arrangement as defined in claim 1,

means for establishing a constant magnetic field along the direction of travel of said annular electron beam thereby to maintain said beam in focus as it passes down through an evacuated drift space. 

1. Apparatus for producing a high power annular pulsed electron beam comprising, in combination, a cathode having a generally conicAl shape with its apex portion blunted, said cathode being of solid construction and made of a nonmagnetic, metallic material; an anode having a frusto-conical shape symmetrically positioned about said cathode such that the longitudinal axis of symmetry of said cathode and said anode are co-extensive; a magnetic coil surrounding said anode; means for coupling a high amplitude current pulse to said magnetic coil, the solid and conducting nature of said cathode preventing any appreciable penetration of this cathode by the pulsed magnetic field thereby produced and causing this magnetic field to be parallel to the slant surface of said cathode in the space between said cathode and said anode; and means for driving said cathode highly negative with respect to said anode when said magnetic field has reached its maximum intensity whereby electrons are emitted from said cathode in a cold emission process and thereafter deflected and focused by said magnetic field into a high-power, annular, pulsed electron beam.
 2. In an arrangement as defined in claim 1 wherein said cathode has a circumferential portion of its outer surface roughened so as to enhance electron emission therefrom when said cathode is driven highly negative with respect to said anode.
 3. In an arrangement as defined in claim 1, wherein said anode is made of thin metallic material so as to permit the magnetic field to penetrate it without distortion.
 4. In an arrangement as defined in claim 1, a cylindrical screen, one end of said screen being connected to the apex portion of said anode and maintained at the same electrical potential as said anode.
 5. In an arrangement as defined in claim 1, means for establishing a constant magnetic field along the direction of travel of said annular electron beam thereby to maintain said beam in focus as it passes down through an evacuated drift space. 